In current wireless communications systems, 5 MHz˜10 MHz radio bandwidths are typically used for up to 100 Mbps peak transmission rate. Much higher peak transmission rate is required for next generation wireless systems. For example, 1 Gbps peak transmission rate is required by ITU-R for IMT-Advanced systems such as the 4th generation (“4G”) mobile communications systems. The current transmission technologies, however, are very difficult to perform 100 bps/Hz transmission spectrum efficiency. In the foreseeable next few years, only up to 15 bps/Hz transmission spectrum efficiency can be anticipated. Therefore, much wider radio bandwidths (i.e., at least 40 MHz) will be necessary for next generation wireless communications systems to achieve 1 Gbps peak transmission rate.
Orthogonal Frequency Division Multiplexing (OFDM) is an efficient multiplexing protocol to perform high transmission rate over frequency selective channel without the disturbance from inter-carrier interference. OFDM has been adopted by both IEEE 802.16m and LTE draft standards and is anticipated to be a foundation of next generation wireless communications systems. Based on OFDM, various multiple access schemes such as OFDMA, OFDM/CDMA, and OFDM/TDMA have been developed and utilized in multi-user wireless systems.
FIG. 1 (Prior Art) illustrates two typical architectures to utilize much wider radio bandwidth for OFDM systems. In a traditional OFDM system, a single radio frequency (RF) carrier is used to carry one wideband radio signal, and in an OFDM multi-carrier system, multiple RF carriers are used to carry multiple narrower band radio signals. In the example of FIG. 1, a traditional OFDM system 1 uses a single RF carrier #1 to carry a wideband radio signal #1, transmitted through one frequency channel #1 (i.e., 40 MHz Bandwidth, 4096 FFT). On the other hand, an OFDM multi-carrier system 11 uses four RF carriers #1-#4 to carry four narrower band radio signals #1-#4, each transmitted through a corresponding 10 MHz frequency channel #1-#4 (i.e., 10 MHz Bandwidth, 1024 FFT).
An OFDM multi-carrier system has various advantages as compared to a traditional OFDM system. First, an OFDM multi-carrier system has lower Peak to Average Power Ratio (PAPR) for uplink transmission because of smaller FFT size for each carrier. Second, it is easier to support backward compatibility with legacy OFDM systems. For example, the frequency channels in an OFDM multi-carrier system are partitioned into 10 MHz bandwidth to fit legacy WiMAX systems. Third, current hardware design such as legacy PHY layer design can be better reused by the same frequency channel bandwidths and parameters. Finally, in an OFDM multi-carrier system, it is possible to have more flexibility in Mobile Stations (MSs) that support different number of carriers and perform different level of service capabilities. Because of such advantages, OFDM multi-carrier systems have become the baseline system architecture in IEEE 802.16m and LTE-Advanced draft standards to fulfill IMT-Advanced system requirements. It is thus desirable to provide a unified network entry procedure to enable the operation of OFDM multi-carrier systems.